The present invention relates to a wound-type induction machine and a variable-speed electromechanical energy converter employing the same.
A conventional wound-type induction machine comprises a rotor formed by fitting an insulated winding (secondary winding) in circular or square, semiclosed or open slots formed in a laminated core formed by laminating silicon steel sheets or a solid core, and a stator formed by fitting an insulating winding (primary winding) in circular or square, semiclosed or open slots formed in the circumference of a laminated core formed by laminating silicon steel sheets and spaced by a gap from the rotor.
Japanese Patent Laid-open No. Sho 63-43598 discloses a variable-speed electromechanical energy converter having an exciter, such as a cycloconverter or a combination of an inverter and a converter, connected to a terminal which is connected through a sliding mechanism, such as a slip ring mechanism, to the secondary winding of a conventional wound-type induction machine.
Japanese Patent Laid-open No. Hei 7-213098 discloses a variable-speed power generating system having an ac system connected to the primary side of the induction generator, and a power rectifier comprising diodes connected to the secondary side of the induction generator, as well as a method of controlling the power generation of the induction generator under the control of the secondary current.
Japanese Patent Laid-open No. Hei 4-334953 discloses an induction motor comprising a stator provided with a stator winding, a first cage rotor provided with a second stator winding, conductors and a rotor shaft, and a second cage rotor provided with conductors and a rotor shaft separate from that of the first cage rotor and disposed coaxially with the first cage rotor.
Japanese Patent Laid-open No. Hei 8-103058 discloses an induction motor comprising a stator provided with a stator winding, a cage rotor provided with secondary conductors and fixedly mounted on a shaft, and a permanent-magnet rotor provided with permanent magnets and rotatably supported on a shaft and disposed coaxially with the first cage rotor.
The conventional technique needs, as an exciter to be connected to the secondary winding of a wound-type induction machine, a power converter capable of controlling the phase angle and the frequency of excitation voltage to be applied to the secondary winding, such as a cycloconverter or an inverter. Moreover, the power converter is expensive and the control method is complicated because the waveforms of voltage applied to and a current supplied to the secondary winding must be sinusoidal waveforms having little harmonic content to prevent the distortion of the primary current, and the main circuit of the power converter employs a switching device capable of optionally determining on-off timing.
Accordingly, it is an object of the present invention to provide a wound-type induction machine which is capable of generating electric power of a satisfactory quality having an output voltage with a reduced distortion.
Another object of the present invention is to provide a variable-speed electromechanical energy converter comprising a wound-type induction machine and a power converter, having a small construction and providing an output including less harmonic content.
According to one aspect of the present invention, a wound-type induction machine comprises a wound stator, a wound rotor disposed inside of the wound stator and fixed to a rotor shaft disposed in alignment with the center of rotation of the wound stator, and a cage rotor disposed between the wound rotor and the rotor shaft.
Even if the current flowing through the secondary winding of the wound stator has a harmonic content, the harmonic content of the current flowing through the secondary winding is attenuated and the harmonic content included in the ac power regenerated from the secondary winding in the ac system connected to the primary winding can be reduced because an induced current flows through the conductors of the cage rotor so as to nullify the revolving magnetic field created in the wound rotor by the harmonic content included in the current flowing through the secondary winding. Since the harmonic content of the current flowing through the secondary winding is attenuated, a current induced in the primary coil by the harmonic content of the current flowing through the secondary winding can be reduced. Therefore, even if the current flowing through the secondary coil has a harmonic content, the distortion of the current flowing through the primary winding can be reduced. Consequently, the outflow of harmonic power from the variable-speed electromechanical energy converter can be suppressed and good-quality electric power of an output voltage that is not distorted significantly can be generated.
According to another aspect of the present invention, a variable-speed electromechanical energy converter employs a wound-type induction machine comprising a wound stator, a wound rotor disposed inside of the wound stator and fixed to a rotor shaft disposed in alignment with the center of rotation of the wound stator, and a cage rotor disposed between the wound rotor and the rotor shaft.
Even if a high-frequency current generated by a power rectifier flows through the secondary winding, a harmonic content included in the current flowing through the secondary winding is attenuated because an induction current flows through the conductors of the cage rotor so as to nullify a revolving magnetic field created in the wound rotor, and the harmonic content included in the ac power regenerated in an ac system connected through the secondary winding, the power rectifier, a chopper, a capacitor and an inverter to the primary winding can be reduced. Since the harmonic content included in the current flowing through the secondary winding is attenuated, a current induced in the primary winding by the harmonic content of the current flowing through the secondary winding can be reduced. Accordingly, the distortion of the current flowing through the primary winding can be reduced even if the current flowing through the secondary winding has a harmonic content, the outflow of harmonic power from the converter can be suppressed, and good-quality electric power of an output voltage that is not distorted significantly can be generated.
The variable-speed electromechanical energy converter of the present invention employs a diode rectifier, or one chopper and a regenerative inverter as the power rectifier. Therefore, even though the converter is simple in configuration and inexpensive, good-quality electric power of an output voltage that is not distorted significantly can be generated.
When the number of the conductors of the cage rotor is greater than the number of the slots in which the primary winding of the wound stator is fitted, leakage reactance of the cage rotor is far lower than that of the primary winding, and the current induced by a current of higher harmonics in the primary winding is far lower than the current induced in the conductors of the cage rotor. Therefore, the distortion of the current flowing through the primary winding by the current induced by the harmonic content can be reduced and good-quality electric power of an output voltage that is not distorted significantly can be generated.
When the cage rotor is provided with permanent magnets, magnetic flux in the machine increases due to the magnetic flux of the permanent magnets. Consequently, the exciting current component of the current flowing through the primary winding of the wound stator is supplemented, and magnetic flux is secured in the machine even if the gap length is great and efficient power generation is achieved. It is desirable when the number of the permanent magnets is equal to an integral multiple of the number of poles of the wound-type induction machine.